handcycles versus wheelchairs, pros and cons for racers

According to results printed in The Boston Globe, in the 2014 Boston Marathon, one handcycle racer, Samuel Spencer, completed the race faster than the winner in the push rim wheelchair division, Ernst Van Dyk (who has won the wheelchair division of this marathon many times.)

This made me curious, wouldn't handcycles, with gearing, have a natural advantage over wheelchairs, which I suspect all lack gearing? (As you can tell, I basically know nothing about this subject.)

Are there reasons other than potential for speed, that marathon competitors have more often ridden racing wheelchairs than they have ridden racing handcycles? Is it simply that racing-designed wheelchairs have been around longer?